Display elements having a light-emitting portion and display apparatuses having such display elements are widely known. For example, a display element (hereinafter, also simply abbreviated as an organic EL display element) having an organic electroluminescence light-emitting portion using the electroluminescence (hereinafter, also abbreviated as EL) of an organic material has attracted attention as a display element capable of emitting light with high luminance through low-voltage DC driving.
Similarly to a liquid crystal display, for example, in a display apparatus (hereinafter, also simply abbreviated as an organic EL display apparatus) including organic EL display elements, a simple matrix type and an active matrix type are widely known as a driving type. The active matrix type has a disadvantage that the structure is complicated but has an advantage that the luminance of an image can be enhanced. The organic EL display element driven by an active matrix driving method includes a light-emitting portion constructed by an organic layer including a light-emitting layer and a driving circuit driving the light-emitting portion.
As a circuit driving an organic electroluminescence light-emitting portion (hereinafter, also simply abbreviated as a light-emitting portion), for example, a driving circuit (referred to as a 2Tr/1C driving circuit) including two transistors and a capacitor is widely known from JP-A-2007-310311 and the like. The 2Tr/1C driving circuit includes two transistors of a writing transistor TRW and a driving transistor TRD and one capacitor C1, as shown in FIG. 3.
The operation of the organic EL display element including the 2Tr/1C driving circuit will be described in brief below. As shown in the timing diagram of FIG. 19, a threshold voltage cancelling process is performed in period TP(2)3 and period TP(2)5. Then, a writing process is performed in period TP(2)7 and a drain current Ids flowing from the drain region of the driving transistor TRD to the source, region flows in the light-emitting portion ELP in period TP(2)8. Basically, the organic EL display element emits light with a luminance corresponding to the product of the emission efficiency of the light-emitting portion ELP and the value of the drain current Ids flowing in the light-emitting portion ELP.
The operation of the organic EL display element including the 2Tr/1C driving circuit will be described later in detail with reference to FIG. 19 and FIGS. 20A to 25.
In general, in a display apparatus, the luminance becomes lower as the operating time becomes longer. In the display apparatus using the organic EL display elements, the fall in luminance due to an temporal variation in a gradation value indicating a black level is observed. Therefore, in the display apparatus, when a single pattern is displayed for a long time, a so-called burn-in phenomenon where a variation in luminance due to the displayed pattern is observed or the like may occur. For example, as shown in FIG. 28A, the display apparatus is made to operate for a long time in a state where characters are displayed (in white) on the upper-right part of a display area EA of the organic EL display apparatus and all areas other than the characters are displayed in black. Thereafter, when the entire display area EA is displayed in white, the luminance of the upper-right part in which the characters have been displayed in the display area EA is relatively lowered as shown in FIG. 28B, which is recognized as an unnecessary pattern. In this way, when the burn-in phenomenon occurs, the display quality of the display apparatus is lowered.